1. Field of the Invention
The invention relates generally to locating an interface between water and hydrocarbon in a subsurface reservoir from within a wellbore. More specifically, the invention relates to detecting changes over time of the interface relative to a wellbore using azimuthally sensitive electromagnetic measurements while drilling or tripping. These changes may be located in any direction relative to the borehole.
2. Background Art
Wellbores are drilled through subsurface rock formations for, among other purposes, making a conduit to enable flow of fluids such as oil and gas to the earth's surface from rock formations which contain such materials. It is frequently the case that oil and gas bearing formations have water bearing zones located below the oil and/or gas. In drilling wellbores through such formations, it is known in the art to drill wellbores directionally, that is, at inclined angles with respect to vertical, even horizontally, to expose as much of the oil and/or gas bearing portion of the formation to the wellbore. It is important when drilling such wellbores to maintain a selected vertical distance between the wellbore and the boundary of the water bearing portion of the formation. Maintaining such distance reduces the possibility of having the wellbore unintentionally penetrate the water bearing portion of the formation, thus reducing the oil and/or gas productive capacity of the wellbore.
U.S. Pat. No. 7,382,135 issued to Li et al. and assigned to the assignee of the present invention describes a method and apparatus for drilling a directional wellbore so as to determine a distance and direction between a wellbore and an electrical resistivity contrasting feature in subsurface formations. Frequently, water bearing formations have substantially different electrical resistivity than oil and/or gas bearing formations, and so the technique and apparatus disclosed in the Li et al. patent is known to be used to maintain wellbore trajectory during drilling so that a distance to a water boundary is maintained. Alternatively, the method and apparatus disclosed in the Li et al. patent may be used to determine distance to a water boundary along a wellbore during “tripping” or other movement of a pipe string in a wellbore.
It is also known in the art that as an oil and/or gas bearing formation located above a water boundary in the same formation has oil and/or gas withdrawn therefrom through one or more wellbores, the position of the water boundary may move upwardly toward the one or more wellbores. Such movement may not be uniform over the entire rock formation because of differences in production rates from each of the one or more wellbores, differences in fluid mobility within the formation and changes in relative permeability as a result of formation subsidence, among other causes. There is a need, therefore, for a technique to determine the position of water boundaries over time, so that subsequent wellbores may avoid being drilled too close to the water boundary, and to be able to adjust production rates and intervals in existing wellbores to optimize extraction of oil and/or gas from the rock formation.